Observation of Electron-Antineutrino Disappearance at Daya Bay

The short-baseline reactor strategy

At baselines of ~1 km from a reactor, ${\bar{\nu }}_e$ oscillations are dominated by the fast $\Delta m_{31}^2$ frequency, giving $P({\bar{\nu }}_e\to {\bar{\nu }}_e)\approx 1-{\sin }^{2}2{\theta }_{13}{\sin }^2\left(\frac{\Delta m_{31}^{2}L}{4E}\right)$ to leading order. The ${\theta }_{13}$ amplitude is therefore directly accessible as a small deficit in the reactor flux at $\sim 1$ km. The theoretical expectation from global fits before 2012 was ${\sin }^{2}2{\theta }_{13}\lesssim 0.1$ — small, but within reach of a sufficiently large detector complex.

The Daya Bay design

Daya Bay uses three reactor complexes (Daya Bay, Ling Ao, Ling Ao-II) in southern China, with combined thermal power of 17.4 GW. The detector system comprises six functionally identical antineutrino detectors deployed in three underground halls: two near halls (~400–500 m baseline) and one far hall (~1600 m average baseline).

Each detector is a cylindrical assembly of 20 tons of gadolinium-loaded liquid scintillator, surrounded by a non-loaded scintillator gamma-catcher and an outer water Cherenkov veto. The gadolinium captures neutrons in ~30 μs with an 8 MeV gamma cascade, giving a sharper delayed-coincidence signature than pure hydrogen capture.

The detectors were constructed to be identical within tight tolerances so that near-to-far ratios cancel systematic uncertainties in reactor flux, detection efficiency, and cross-section — reducing the systematics on ${\theta }_{13}$ extraction to well below the statistical precision.

The first result

The 2012 paper reports 55 days of data with the first two near-hall and far-hall detectors running. The far-to-near ratio deviated from unity at $R=0.940\pm 0.011\pm 0.004$ 5.2σ below unity. The fit gave: ${\sin }^{2}2{\theta }_{13}=0.092\pm 0.016\pm 0.005$ — a definitive first measurement of the smallest mixing angle.

Confirmation and refinement

Daya Bay’s result was confirmed within weeks by the RENO experiment in Korea, and shortly after by Double Chooz in France. By 2013 all three reactor experiments had converging results. Daya Bay has since completed its full exposure; current world-average values (dominated by Daya Bay statistics) are ${\sin }^2{\theta }_{13}\approx 0.0223\pm 0.0007$ — a per-cent-level precision that makes ${\theta }_{13}$ the best-measured neutrino mixing angle.

Why θ₁₃ matters

Before Daya Bay, the possibility of CP violation in the lepton sector was uncertain: if ${\theta }_{13}$ had been zero, the Dirac phase ${\delta }_{CP}$ would be unphysical (every appearance probability would vanish). Daya Bay’s non-zero measurement established that CP violation is accessible to long-baseline accelerator experiments.

The subsequent measurements of ${\delta }_{CP}$ at T2K and NOvA all use the reactor measurement of ${\theta }_{13}$ as an external input, effectively anchoring the ${\theta }_{13}$-${\delta }_{CP}$ degeneracy that would otherwise plague long-baseline appearance analyses. DUNE and Hyper-K will continue this practice.

Significance

The Daya Bay result is the last “major angle” measurement in the PMNS matrix. With all three mixing angles now non-zero and measured, and with two squared-mass differences known, the remaining frontier is the mass ordering and the CP-violating phase — both under active experimental pursuit.